Ukraine again accused pro-Russian separatists Monday of violating the cease-fire agreement the two sides signed in Minsk, Belarus last month, saying the rebels had fired rockets and artillery at Ukrainian military positions in eastern Ukraine.

Ukrainian military spokesman Andriy Lysenko said six soldiers were wounded in fighting over the previous 24 hours. He said the separatists had fired Grad missiles at government positions near the village of Orikhove, and fired mortars at other positions.

On Saturday and Sunday, the Ukrainian military accused the rebels of shelling its forces in various parts of eastern Ukraine.

NATO officials and European monitors have confirmed the renewed fighting, with the head of the Organization for Security and Cooperation in Europe, Ivica Dacic, calling the new violence -- particularly near the Ukraine port city of Mariupol -- "unacceptable."

Separately, a NATO military official cited the cross-border transfer of more than 1,000 Russian heavy weapons into Ukraine territory in recent weeks.

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Meanwhile, a rift appears to be widening between Ukrainian President Petro Poroshenko's administration and one of its key allies -- billionaire regional governor Ihor Kolomoisky.

The controversy erupted late last week, when Kolomoisky and a group of armed men showed up at the Kyiv headquarters of the state oil pipeline operator Ukrtransnafta, which had fired a key Kolomoisky ally.

Kolomoisky -- who backs Kyiv and has bankrolled volunteers fighting against Russia-backed rebels -- left the Ukrtransnafta headquarters peacefully after several hours, along with his armed entourage.

Responding to the incident Monday, Poroshenko praised the service of the volunteer units, but said private armies would not be permitted in the country. A short while later, Ukrainian Interior Minister Arsen Avakov ordered all private security services to "unambiguously bring their activity impeccably in line with the letter of the law" within 24 hours.

Separately, the head of Ukraine's State Security Service, Valentyn Nalyvaychenko, accused officials in Kolomoisky's administration in the Dnipropetrovsk region of financing criminal gangs and threatening investigators.